Neuropathic pain is thought to occur because of a sensitization in the peripheral and central nervous systems after an initial injury to the peripheral system. Direct injury to the peripheral nerves as well as many systemic diseases including AIDS/HIV, Herpes Zoster, syphilis, diabetes, and various autoimmune diseases, can induce this disorder. Neuropathic pain is typically burning, shooting, and unrelenting in its intensity and can sometimes be more debilitating that the initial injury or the disease process which induced it. Unfortunately, the few remedies that have been reported to alleviate this condition is effective in only a small percentage of patients.
Capsaicin, a pungent substance derived from the plants of the Solanaceae family (hot chili peppers) has long been used as an experimental tool because of its selective action on the small diameter afferent nerve fibers, or C fibers, that are believed to mediate pain. From studies in animals, capsaicin appears to trigger C fiber membrane depolarization by opening cation selective channels for calcium and sodium. Although detailed mechanisms are not yet known, capsaicin mediated effects include: (i) activation of nociceptors in peripheral tissues; (ii) eventual desensitization of peripheral nociceptors to one or more stimulus modalities; (iii) cellular degeneration of sensitive unmyelinated C fiber afferents; (iv) activation of neuronal proteases; (v) blockage of axonal transport; and (vi) the decrease of the absolute number of C fibers without affecting the number of myelinated fibers.
Because of capsaicin's ability to desensitize nociceptors in peripheral tissues, its potential analgesic effects have been assessed in various clinical trials. However, since the capsaicin application itself frequently causes burning pain and hyperalgesia apart from the neuropathic pain being treated, patient compliance has been poor and the drop out rates during clinical trials have exceed fifty percent. The spontaneous burning pain and heat hyperalgesia are believed to be due to intense activation and temporary sensitization of the peripheral nociceptors at the site of capsaicin application (primary hyperalgesia). Mechanical hyperalgesia evident in areas surrounding the site of topical application appears to originate from central sensitization of dorsal horn neurons involved in pain transmission (secondary hyperalgesia). Because of these side effects, the maximal capsaicin concentration used in previous human studies has been limited to 0.075%.
Analogs of capsaicin with similar physiological properties are known. For example, resiniferatoxin is described as a capsaicin analog by inventor Blumberg, U.S. Pat. No. 5,290,816, issued Mar. 1, 1994. Inventor Brand in U.S. Pat. No. 4,812,446, issued Mar. 14, 1989, describes capsaicin analogs and methods for their preparation. Further, inventors LaHann et al. in U.S. Pat. No. 4,424,205, issued Jan. 3, 1984, cite Newman, "Natural and Synthetic Pepper-Flavored Substances" published in 1954 as listing pungency of capsaicin-like analogs. Ton et al., British Journal of Pharmacology, 10, pp. 175-182 (1955) discuss pharmacological actions of capsaicin and its analogs.